1. Field of the Invention
The present invention relates to a method of handoff control and a handoff control system in a cellular communication system and, more particularly, to a method of handoff control and a handoff control system which perform handoff processing by assigning a priority to each request for handoff from a mobile terminal.
2. Description of the Related Art
In recent years, the number of subscribers in mobile communication systems is unceasingly on the increase, and hence an increase in subscriber capacity is required. Reducing each cell radius in the cellular communication system is proposed as one of solutions for increasing the subscriber capacity. Reducing the cell radius means that a total number of base stations in the service area of the cellular communication system increases. As a consequence, the number of channels available for subscribers can be increased.
On the other hand, reducing the cell radius causes a mobile terminal to increase a chance of handoff operation. FIG. 11(A) is a schematic view for explaining a handoff operation in a general cellular communication system; and FIG. 11(B) is a view showing a case wherein the cell radius is reduced.
In FIG. 11(A), assume that the mobile terminal 110 is locating in the cell 130a of the base station 120a and is communicating with the base station 120a, and the mobile terminal 110 is moving into the cell 130b of the base station 120b. As the mobile terminal 110 is moving away from the base station 120a, the reception signal intensity from the base station 120a gradually decreases at the mobile terminal 110. In this case, the reception signal intensity indicates the magnitude of signal power received from the base station.
The reception signal intensity from the base station 120a is periodically measured by the mobile terminal 110. When the measured reception signal intensity becomes equal to or less than a predetermined threshold level, the mobile terminal 110 sends a handoff request to the base station 120a for continuing the present communication of a call by changing the base station from the base station 120a to the base station 120b. With this operation, the mobile terminal 110 is set in a state in which it can also receive service from the base station 120b. 
The handoff request sent from the mobile terminal 110 to the base station 120a is notified from the base station 120a to the base station 120b through a network apparatus such as a base station control or mobile switching center equipment (not shown). Thereafter, the mobile terminal 110 can communicate with both the base stations 120a and 120b. When the mobile terminal 110 is further moving away from the base station 120a, the communication with the base station 120a is disconnected, and the mobile terminal 110 only communicate with the base station 120b. 
An area where the mobile terminal 110 can communicate with both the base stations 120a and 102b is an overlap area 140 where the cells 130a and 130b overlap.
When the cell radius is reduced as shown in FIG. 11(B), the above handoff operation is frequently performed. For this reason, as the cell radius decreases, the handoff traffic increases, and forced termination of communication tends to occur due to lack of communication channels.
A method of avoiding such a situation by preparing a queue for handoff requests by each of mobile terminals is disclosed in, for example, D. Hong et al., xe2x80x9cTraffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff proceduresxe2x80x9d, IEEE Trans. Veh. Technol., vol. VT-35, August 1986 (reference 1) and Q. A. Zeng et al., xe2x80x9cPerformance analysis of mobile cellular radio system with priority reservation handoff proceduresxe2x80x9d, IEEE Proc. VTC-94, vol. 3, June 1994 (reference 2).
According to reference 1, of all the set channels, some number of channels are always reserved as handoff channels, and the dedicated handoff channels are only used for the handoff operation, and not used for ordinary new calls. With this arrangement, the call loss probability during handoff operation due to lack of communication channels is reduced. According to reference 2, a buffer for new calls is provided in addition to the technique disclosed in reference 1 to decrease the call loss probability for new calls without increasing the blocking probability and probability of forced termination of handoff calls much.
In addition, Japanese Patent Laid-Open No. 7-264656 (reference 3) discloses a technique of assigning priority to each handoff process on the basis of a calculation result considering the moving speed and direction of each mobile terminal, and performing the handoff processes in accordance with the priority.
Each mobile terminal is moving across cells at various speeds. For example, the moving speed of a mobile terminal that is moving on a car differs from that of a mobile terminal carried by a walking user. As the moving speed of each mobile terminal differs in this manner, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes. When handoff processes are performed only in the order of accepting handoff requests, forced termination of communication may occur at a certain mobile terminal that is moving at a high speed if between the instant at which a handoff request is generated and the instant at which the handoff process is completed is prolonged.
In addition, a mobile terminal that moves at a high speed generates handoff requests many times during one communication because it passes many cells in a predetermined period of time, and hence the probability of forced termination of communication tends to increase. Furthermore, mobile terminals move across cells through various routes; some mobile terminals move away from the base station from which service is provided, and some mobile terminals move while keeping distances to the base station constant.
When mobile terminals move through different routes as described above, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes as well. When handoff processes are simply performed in the order of handoff requests, a delay in performing a handoff process may cause forced termination of communication at a mobile terminal which moves away rapidly from the base station currently under communication.
In the technique disclosed in reference 3, since complicated arithmetic operation is required to assign priorities, and a priority is assigned to each call, processing for a handoff request is frequently performed and complicated.
It is an object of the present invention to provide a handoff control system and method which can decrease the probability of forced termination of communication by considering the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed.
In order to achieve the above object, according to the present invention, a handoff control system for performing a handoff processing for a mobile terminal, that moves across cells of a plurality of base stations while performing mobile communication, comprises the following mobile terminal and a base station.
A mobile terminal which periodically measures a reception signal intensity of a radio signal used for a current communication, calculates a relative change value of reception signal intensity at measurement time intervals, and
periodically reports the measured reception signal intensity and the calculated relative change value of reception signal intensity; and
a base station which periodically receives and stores the reception signal intensity and the relative change value of reception signal intensity respectively reported from the mobile terminal, and performs a handoff processing by giving a higher priority to a handoff request of a mobile terminal for which the stored relative change value of reception signal intensity is larger and the stored reception signal intensity is weaker.
In more detail, the mobile terminal comprises a reception signal measuring section which periodically measures a reception signal intensity of a radio signal used for a current communication, a relative change value calculating section which calculates a relative change value of reception signal intensity output from the reception signal measuring section at measurement time intervals, and a control section which reports the measured reception signal intensity and the calculated relative change value of reception signal intensity periodically to the base station under communicating, and outputs a handoff request when the measured reception signal intensity has reached a predetermined handoff threshold level. Also, the base station comprises a memory table which stores data of the reception signal intensity and the relative change value of reception signal intensity in association with each mobile terminal, and renews data in accordance with each periodical report, a plurality of queues to which priories based on relative change values of reception signal intensity are assigned in advance, a handoff request processing section which distributes a handoff request from each mobile terminal to one of the queues on the basis of the relative change value of reception signal intensity corresponding to the mobile terminal stored in the memory table, a queue sequence determination section which determines handoff processing orders for a plurality of handoff requests distributed in the same queue with giving a higher priority to a handoff request for which corresponding reception signal intensity stored in the memory table is weaker, and a queue control section which performs a handoff processing for a handoff request having a higher processing order waiting in the queue having a higher priority.
In the present invention, the relative change value calculating section can be equipped in the base station instead of equipped in the mobile terminal. In this case, the mobile terminal can only report the reception signal intensity to the base station, and the relative change value calculating section in the base station calculates the relative change value of the reception signal intensity by using periodically reported reception signal intensity from the mobile terminal.
According to the present invention, a method of handoff control for performing a handoff processing for a mobile terminal, that moves across cells of a plurality of base stations while performing mobile communication, comprises the following elements:
measuring a reception signal intensity of a radio signal used for a current communication periodically at said mobile terminal;
calculating, at the mobile terminal, a relative change value of reception signal intensity by the measured reception signal intensity at measurement time intervals;
reporting the measured reception signal intensity and the calculated relative change value of reception signal intensity periodically from the mobile terminal to the base station under communicating;
receiving and storing, at the base station, the reception signal intensity and the relative change value of reception signal intensity respectively reported from the mobile terminal;
sending a handoff request by the mobile terminal when the measured reception signal intensity has reached a predetermined handoff threshold level; and
performing a handoff processing at the base station by giving a higher priority to a handoff request of a mobile terminal for which the stored relative change value of reception signal intensity is larger and the stored reception signal intensity is weaker.
Also in this method invention, the calculation of the relative change value of reception signal intensity can be performed in the base station instead of performed in the mobile terminal. That is, the mobile terminal only reports the measured reception signal intensity periodically to the base station under communicating, and in the base station, it receives the reception signal intensity, calculates a relative change value of reception signal intensity by the reception signal intensity at reporting time intervals, and stores the reception signal intensity and the relative change value of reception signal intensity respectively in association with the mobile terminal.
The element of performing a handoff processing at the base station further comprises the following elements:
distributing a handoff request from each mobile terminal to one of plurality of queues provided in the base station, to which priories based on relative change values of reception signal intensity are assigned in advance, on the basis of the relative change value of reception signal intensity corresponding to the mobile terminal stored in the memory table:
determining handoff processing orders for a plurality of handoff requests distributed in the same queue with giving a higher priority to a handoff request for which corresponding reception signal intensity stored in the memory table is weaker: and
performing a handoff processing for a handoff request having a higher processing order waiting in the queue having a higher priority.